Electrical Circuit Breakers

Electrical Circuit Breakers

By: L. W. Brittian, Mechanical-Electrical Instructor

PART 2

IN THIS THE SECOND PART OF THE ARTICLE COVERING CIRCUIT BREAKERS, THE FOLLOWING TOPICS ARE COVERED:

• FUNCTIONS
• TYPES
• COMPONENTS
• VOLTAGE RATINGS
• AMPERE RATINGS
• AMPERE INTERRUPTING CAPACITY (AIC)
• TESTING-LISTING OF CIRCUIT BREAKERS
• NOT ALL “BREAKERS” ARE RATED THE SAME
• THE ELECTRICAL ARC
• EFFECTS OF CURRENT FLOW
• THERMAL ENERGY
• THERMAL TRIP ELEMENT
• MAGNETIC TRIP ELEMENT
• HYDRAULIC-MAGNETIC TRIP ELEMENTS

CIRCUIT BREAKER FUNCTIONS

Later in this article we will cover several more specific features of some specialized types of circuit breakers, but for now let us begin by saying that a circuit breaker’s main functions are:

Sense the current flowing in the circuit
Measure the current flowing in the circuit
Compare the measured current level to its pre-set trip point
Act within a predetermined time period by opening the circuit as quickly as possible to limit the amount of energy that is allowed to flow after the trip point has been reached.

In a condensing manner, we can say that a circuit breaker functions to provide overcurrent protection, and isolation from energized and un-energized circuit components. As safety devices, these functions must be performed, without failure, without damage to the protected circuit’s components, from no current through the breaker’s rated ampere interrupting capacity (AIC). Now that is a big job and an important job.

Modern breakers routinely do their job day in and day out with very little maintenance. Like all things that are made by man, they do have limits and they do fail. Hopefully this paper will help you to better understand and appreciate the task performed by those little black boxes.

CIRCUIT BREAKER TYPES

Medimum and low voltage circuit breakers are commonly separated into the following groups based upon the type of material used to make the frames or cases:
• Molded case, (MCCB) the most common low voltage type
• Insulated case, (ICCB) the intermediate voltage and amperage sizes
• Metal clad, the higher in voltage (medium) and amperage rating

CIRCUIT BREAKER COMPONENTS

The five basic components of a circuit breaker are:
• Frame or case made of metal or some type of electrical insulation
• Electrical contacts
• Arc extinguishing assembly
• Operating mechanism
• Trip unit, containing either a thermal element, a magnetic element or both

CIRCUIT BREAKER VOLTAGE RATINGS

Low voltage (under 600 volts) circuit breakers are commonly rated for, 120 volts, 240 volt, 277 or 480 volts AC. Some breakers are rated for used in DC circuits, while others are rated for use in either AC or DC circuits.

Single pole circuit breakers are rated for a voltage potential between the one hot wire and a grounded surface. Breakers that are intended to be part of a two or three phase circuit are rated for a voltage potential from opposite potential, to opposite potential, or phase-to-phase. You must not use two single pole 240 volt breakers to control a 480 volt circuit, but two single pole breakers rated 277 volts could be used to control a 240 volt circuit.

When improperly applied outside of its rating, a breaker may not be able to extinguish the arc when attempting to clear a fault. Some breakers have what is called a slash (/) rating such as 120/240 or 277/480. Breakers that are slash rated should not be used on un-grounded systems, as they have not been tested for safe operation on these types of systems. For a more detailed coverage of this topic review Mr. Holt’s article “Understanding Circuit Breaker Markings,” in the November 2001 issue of EC&M magazine. Cooper-Bussmann also has an article covering slash rated circuit breakers, if you would like to read still more.

CIRCUIT BREAKER AMPERE RATINGS

Circuit breakers have an ampere rating (typically marked on the end of the operating handle). This is the maximum continuous current that the breaker can carry without exceeding its rating. As a general rule the circuit breaker’s ampere rating should be the same as the conductor’s ampacity. In other words we would not want to put a 60 amp breaker on a 10 amp wire. Breakers are tested in open air, with a temperature of some 40 or 50 degrees C.

When a breaker is placed within an enclosure, cooling airflow is restricted; this reduces the ability of the breaker to carry a current to 80% of its ampere rating. When they are installed in an electrical enclosure, breakers will trip when a current in the amount of their rating is placed upon them continuously. Breakers are designed to be able to safely carry a current in excess of their rating for very very short periods of time to allow some types of electrical equipment (called inductive loads) such as motors to start up.

While not as common, some breakers are rated for 100% continuous loads. These are typically called supplementary protectors (SP) and not circuit breakers.

AMPERE INTERRUPTING CAPACITY (AIC)

Circuit breakers are tested and then rated as to their ability to open the protected circuit with a specific amount of current flowing in the circuit. Circuit breakers typically have AIC ratings of between 5,000 and 200,000 AIC. The amount of fault current available must not exceed the breaker’s ability to safely open the circuit. Not only must the breaker be rated for the applied voltage, and continuous amperage load; it must also have an AIC rating equal to or greater than the available current at the location in the circuit where it will be installed. Breakers that have been installed so that the available fault current exceeds its AIC rating may blow up, just like a bomb would explode were it to attempt to clear a fault current above its rating. When opening a faulted circuit, it is possible for smoke and fire to be exhausted from a breaker. If you would like to see a breaker belch fire and smoke, see if you can locate and view the Cooper-Bussmann fuse company videotape titled “Specification Grade Protection”. The visual impact of this tape will likely enhance your appreciation of the importance of an electrical device’s AIC rating far better than any words of mine.

In your safety classes, you likely have received training in the step to the side routine before manually switching electrical circuits, and this videotape will reinforce the value of this easy safety step. This is also a good reason why sheet metal covers called dead front trim should be re-installed on loadcenters, panelboards, and the like before operating switching devices.

Electrical engineers tell us that the two major factors that govern the amount of fault current that can be delivered in a system are the KVA rating of the transformer and the impedance of the transformer. The presence of connected electric motors in the circuit also adds to the amount of potential fault current. Considering 480 volt systems, combined transformer and motor fault currents can range from 14,400 amps for a 500 KVA transformer with an impedance of 5.0% to some 90,000 amps for a 3500 KVA transformer with 5.75% impedance. Selecting all circuit breakers for higher AIC ratings may be the safety first and cost last method.

An engineering level study of a facility’s electrical system every five years (or before plant remodeling is undertaken) is a good idea. The study should include a review of the AIC of the plant’s breakers and the fault current that the plant’s electrical circuits can deliver to the line terminals of all major circuit breakers (OCPD).

TESTING-LISTING OF CIRCUIT BREAKERS

Molded case low voltage circuit breakers are typically tested to UL standard 489. UL uses the following test goals to determine if a breaker is considered to be safe (incompliance with their safety standard):

The breaker must interrupt the maximum short circuit current two times.
The breaker must protect itself and the connected conductor and the equipment it is installed in.
After having been tested the breaker must be fully functional and pass a thermal calibration trip test at 250% of its rated ampacity; and pass a dielectric withstand test at two times its rated voltage or a minimum of 900 volts.
The tested breaker must also operate properly and have continuity in all of its poles.

UL-489 listed circuit breakers are tested with a four-foot length of wire, as they must perform during the test as they would when installed in the real world, so wire is connected to make the test a bit more realistic. During the test the conductor’s insulation must not be damaged. The connected wires must not be pulled loose from the breaker-conductor termination lug. The breaker case must not be damaged as a result of cable whip forces (caused by the potentially huge amount of magnetic force developed under short circuit conditions). The connected wire acts to some degree as a heat sink for the breaker. That is, it helps to dissipate heat produced within the breaker. This is because the breaker’s case acts as not only an electrical but a thermal insulation also, in that it tends to retard the rate of heat transfer. This is one reason why breakers have wire size ranges marked on them. Too small a wire attached to the breaker cannot adequately aid in cooling the breaker.

The temperature at the circuit breaker’s terminals must not rise more than 50 degrees C. above the ambient air temperature surrounding the breaker. The UL-489 test standard has been used to test many, many circuit breakers over the years and has proven to be a pretty good standard by which the safety of circuit breakers can be determined.

If you would like information about European standards covering circuit breakers (IEC-947-2), I suggest that you read Cashier technique # 150. Be prepaired for a good bit of concentrating; this document is written at the engineering level. The good folks with Square D can help you locate it on their WEB site.

NOT ALL “BREAKERS” ARE RATED THE SAME

A circuit breaker listed to UL-489 standard is not the same animal as a breaker- looking thing listed to a UL standard as a supplementary circuit protector (SP). A circuit breaker listed to UL standard 489 will open the circuit under fault current conditions and is tested to a higher degree to do so than is a supplementary protector (commonly tested to UL-1077 safety standards).

Supplementary protectors cannot be used as service equipment without there being some device such as a UL-498 listed breaker or fuse in the circuit up-stream of them, as they may or may not open the circuit under short circuit conditions. It may be difficult to determine the difference between a circuit breaker and a supplementary protector by simply looking at an installed device. The good folks with UL have pointed out that we need to pay close attention to what we are working with, as the testing procedures and listing requirements differ among all of these look-a-like black boxes.

I wish that I could pass on some sure fire just looking at it (without using a book or removing the device) method of determing if it was a circuit breaker, or a supplementary protector, but at this moment regrettably I am unable to do so.

The same is somewhat true of magnetic trip only (short circuit protection) motor circuit protectors (MCP). With MCP’s it helps that an amperage rating is not imprinted on the end of the operator handle. However that aid is of limited value, as the NEC allows the marking to be hidden by some type of covering trim when a circuit breaker is rated over 100 amps. (See article 240.83 (A) and (B) for more details). Supplementary protectors are not required to have an AIC marked on them, but neither are circuit breakers that have an AIC of 5,000 amps. If you are a bit confused, so am I; and try as best they can, UL has not been able to communicate to me a hard and fast rule of how I can physically tell the difference in the field without removing the device, or finding part numbers and looking them up in a parts book (that plant maintenance folks do not typically have readily accessible). You can obtain additional information about the listing of Supplementary Protectors by reviewing a copy of UL’s listing guide number QVNU2 and circuit breakers number DIVQ.

THE ELECTRICAL ARC

As soon as two energized electrical contacts separate, one contact (called the cathode) transmits electrons and the other (called the anode) receives them, that is an electrical arc is created. If you were to ask a layman to tell you what electricity looks like, he would likely describe an electrical arc, which it is not. We frequently see a wide range of arcs (the Godzilla of electrical arcs), the lightening strike, and the micron sized static electrical discharge occasionally experienced after walking across a carpeted floor.

The electrical arc is a naturally occurring event a part of doing business with electricity so to speak. The visible arc (ionized air) is not electricity but an effect of electricity, just as heating of conductors when current flows in a circuit. An electrical arc produces an intense amount of heat that can reach temperatures of 4,000 C and higher. If not extinguished quickly, an arc can pit (a transfer of metal from one surface to another) or even destroy the electrical contacts and insulating material such as the breaker’s casing.

Circuit breakers are designed to minimize, if not eliminate, damage caused by electrical arcs in the following ways:

Submerge the contacts in oil
Place the contacts in a vacuum tight enclosure
Immerse the contacts with an inert gas such as SF-6
Divert the arc away from the main contacts to secondary contacts or arc horns
Divert the arc away from the contacts with a magnetic field (blowout coils)
Deflect the arc off of the contacts by use of a differential pressure
Extinguish the arc in arc chutes
Make and separate the contacts at high speed

Low and medium voltage circuit breaker manufactures have used combinations of the above methods. Methods such as oil, vacuum, and gases are less common on modern low and medium voltage breakers.

While it is correct to say that when the AC sine wave reaches the zero voltage points, the arc will go out due to the lack of voltage, this is not the entire picture, for arcs are much more complicated. Quickly stated, the arc has a voltage of its own, and if the air between the contacts is not cooled sufficiently, or the air gap is not wide enough, the arc may re-establish itself when the supply circuit voltage again increases.

A common method used in the above 200 amp or so size breaker is the use of arc extinguishing chutes. This method diverts and separates individual sections of the arc away from the contacts into thermally and electrically conductive chutes where the arc is stretched and cooled sufficiently to extinguish it. The use of contact surface coating material such as silver is used to harden contact surfaces and reduce pitting damage. Spring powered switching contacts are designed to increase contact movement speed to reduce the life of an arc.

While many electrical circuits are wired using copper conductors, copper-only contacts are not used because heating causes a type of corrosion that increases the contact’s impedance, which in turn increases the amount of heat generated.

An arc can travel across some types of insulated surfaces that have been heated so hot as to produce a carbon tract that provides a lower resistance path for future current flow. This means that external breaker insulation materials should be inspected from time to time for indications of overheating, dust, and for the possible formation of a fine carbon-like material trail that can result in a short circuit.

If you would like to read more about the electrical arc at an engineers reading level, then Cahier technique # 154 is a good article. It can be down loaded for free from the WEB. The good folks with Square D can help you locate it on their WEB site.

THE EFFECTS OF CURRENT FLOW

When current flows in a circuit two effects are produced, magnetic and thermal. Thermal energy is comparatively a much slower phenomenon to build up than a magnetic force. For example, under short circuit current conditions, the magnetic forces build up very quickly. Just as a magnetic can be used to move a metal object, so can magnetic forces torque or stress circuit components.

Under severe short circuit current conditions busbars have been instantly ripped from their mountings, and large cables have been whipped so violently as to have been pulled loose from their terminations. At the same time the slower thermal energy was melting sand in fuses into glass, steel and copper metals were being heated so hot as to be turned into a superheated gas (solid metal became a liquid, and then a vapor). These events occurred within the time it took the OCPD to open the faulted circuit. Circuit breakers routinely open shorted circuits in something like 3/60 to 5/60 of one second.

We may tend occasionally to focus our attention on the electrical insulation aspects, while potentially forgetting magnetic and thermal effects under short circuit current conditions. The practice of securing big cables in place so that they stay in place under short circuit current conditions with thin plastic like twine should be reconsidered. So too should the practice of tightening busbar fasteners without the use of a torque wrench.

THERMAL ENERGY

Excessive current flowing in a circuit can result in heat related damage to electrical equipment. That is because a rise in current results in an increase in thermal energy. Mathematically speaking, a current increase results in a squared value increase in the amount of heat, that is, I squared T means that the higher the current the much greater the amount of heat that will be developed. Many years ago it was established that an increase of only twenty degrees C. above the maximum rated temperature of an electrical insulator (motor windings) can reduce its life by as much as 50%. Electrical insulation can withstand only a limited amount of repeated overheating (much the same as structural stress cycles are cumulative) before it fails.

THERMAL TRIP ELEMENT

When the circuit is required to be provided with a protective device for overload type conditions, a thermal time delay element is typically provided. The thermal element provides a time delay function called Inverse. That is to say, as the current flow in the circuit increases, heat begins to builds up in a BI-metal element (that is made from two thin strips of different metal) and it begins to bow and cause the contacts of the breaker to open. These two metals are selected for their different rates of thermal expansion (heating) and contraction (cooling). Having been fused together by the manufacture, changes in their temperature results in them expanding and contracting in an arc, and not in a straight line. This movement allows them to be used as the source of the force needed to open the breaker’s contacts.

Thermal elements require some of the heat to be dissipated before they can be reset after having tripped. This means that when a breaker trips on thermal element (due to a running overload) it may need a few minutes to cool off before it can be reset.

MAGNETIC TRIP ELEMENT

The trip unit is the brain of the breaker. It consists of the components necessary to automatically open the circuit when an overcurrent is sensed. Generally a magnetic sensing element or both a magnetic and a thermal sensing element will be included in the trip unit.

When a breaker has only a magnetic sensing element, it is a non-delay instantaneous trip type. With this type of circuit breaker, no delay has been intentionally designed into its operation. These devices have a magnetic coil that surrounds a moveable plunger, which is held in place by a spring. The circuit current flows through the magnetic coil and when it produces a pull on the plunger greater than the retaining spring, it will move the plunger, which results in the device’s contacts opening.

When an OCPD has only a magnetic sensing element it will provide protection only from short circuit level currents and not from overload level currents. These types of devices are called motor circuit protectors (MCP). They are used when a device such as a three phase motor starter with thermal overload relay-heater elements provides running overload protection.

When a circuit breaker has tripped on the magnetic element, it can be immediately reset. One should not reset a breaker more than twice without correcting the cause of the fault. To do so may result in serious personal injury.

HYDRAULIC-MAGNETIC TRIP ELEMENTS

Some brands of circuit breakers use a hydraulic fluid (silicone) type of current sensing element. With this type of sensor, a wire is coiled around an oil filled cylinder containing a piston, which is connected on one end to the breaker’s trip unit. This forms a magnetic coil through which load current flows. The piston is held in a position by a spring. When current flows in the coil, a magnetic field is created that pulls the piston deeper and deeper into the coil. As the current in the circuit increases, so does the coil’s magnetic field strength; the spring is compressed, drawing the piston deeper into the coil, increasing the coil’s magnetic field. As the plunger movement progresses, the fluid tends to oppose rapid movement of the piston in the cylinder.

By varying the fluid’s viscosity the manufacture can alter the amount of force that retards the piston’s movement; this in turn allows the amount of time delay to be varied. By changing the size of the coil wire and number of wraps of the wire in the coil, the amount of force (MMF) created by the magnetic field can be changed (changing either or both the quantity of amps, or the number of turns of the wire changes the amount of pull produced by a electro-magnetic coil).

Manufactures using this type of element design can offer the protection of a quick responding magnetic element and the time delay of a thermal element in their breakers without using a bi-metal element.

In the next part of this article the following topics will be reviewed:

• Methods Of Mounting Circuit Breakers
• Fixed Mounted Circuit Breakers
• Removable Mounted Circuit Breakers
• Drawout Mounted Circuit Breakers
• Methods Of Securing Circuit Breakers
• Stab Lock Type Circuit Breakers
• Bolted Type Circuit Breakers
• Din Rail Mounted Circuit Breakers

If you have any questions or comments, please send me an E-mail.

Remember Work Smarter, Not Harder
L. W. Brittian
Mechanical-Electrical Instructor
lwbrittian@hot1.net